Accumulated other comprehensive income (loss) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Accumulated Other Comprehensive Income Rollforward [Roll Forward]
Balance as of December 31, 2017	$ 7,067,009
Balance as of March 31, 2018	7,346,009
Foreign currency translation adjustments
Accumulated Other Comprehensive Income Rollforward [Roll Forward]
Balance as of December 31, 2017	(547,927)
Current period other comprehensive income	71,494
Balance as of March 31, 2018	(476,433)
Pensions, net of tax
Accumulated Other Comprehensive Income Rollforward [Roll Forward]
Balance as of December 31, 2017	(10,600)
Current period other comprehensive income	(135)
Balance as of March 31, 2018	(10,735)
AOCI attributable to parent
Accumulated Other Comprehensive Income Rollforward [Roll Forward]
Balance as of December 31, 2017	(558,527)
Current period other comprehensive income	71,359
Balance as of March 31, 2018	$ (487,168)